Vulcanization of dip-molded rubber articles with reduced molten media bath times

ABSTRACT

Pore-free rubber articles are prepared by dip-molding in a dipping medium that includes a vulcanizing agent, then partially-cured by immersing the dip former in a heated liquid bath that is chemically inert. A particularly effective liquid bath is a molten, nitrite free inorganic salt. The partially-cured rubber is then maintained at a desired curing temperature in a low/no oxygen heating oven to complete curing. Alternatively, upon removal from the molten salt bath, the latex film is quenched.

BACKGROUND OF THE INVENTION 1. Field of Invention

The invention lies in the field of dip-molded rubber articles, and moreparticularly to methods of vulcanization of dip-molded rubber articles.

2. Description of Related Art

Natural rubber latex has been extensively used as a material ofconstruction for elastomeric dip-molded medical devices and medicaldevice components. Examples of medical devices and components made fromnatural rubber latex are surgical gloves, examination gloves, fingercots, catheter balloons, uterine thermal ablation balloons, cathetercuffs, condoms, contraceptive diaphragms, indwelling urinary drainagecatheters, and male external urinary drainage catheters. Other exampleswill be apparent to those skilled in medicine and in the manufacture anduse of these and similar medical devices. Dip-molding techniques arealso used in making elastomeric devices for non-medical uses. Theseinclude toy balloons, industrial gloves, household gloves, and othersimilar devices. These devices, both medical and non-medical, can alsobe formed from synthetic rubber latex materials rather than naturalrubber. In some cases, synthetic materials are preferred, for examplewhere natural rubber is deemed unsuitable for some reason, e.g.,allergic reaction, or where the synthetic material offers an advantage.U.S. Pat. No. 6,329,444 to McGlothlin et al., the entire disclosure ofwhich is incorporated herein, addresses the use of syntheticcis-1,4-polyisoprene in making medical and non-medical devices.

In latex dip-molding processes, dip formers are dipped in a latex bath,then withdrawn from the bath, and then typically dried and vulcanized inhot air. Vulcanization is the process of transforming an uncurred rubbercompound into a highly elastic product by forming a three-dimensionalcross-linked network structure in the rubber matrix. In some cases, thelatex is pre-vulcanized; i.e., the rubber particles in the latex arepartially or fully vulcanized prior to the dipping step. A prevulcanizedlatex produces a film with improved wet and dry gel strengths, and whenfurther vulcanization is performed after dipping and hot air drying, thetensile properties are improved. An advantage of prevulcanization is areduction in the process time by lessening or eliminating the timerequired for the post-dip vulcanization. In some dip-molding processes,a chemical coagulant is included in the latex or on the dip former, andheat-sensitized coagulant dipping methods are applied to producearticles having a greater film thickness. Multiple dips are also used insome processes. Vulcanization performed on the latex film after the dipformer is removed from the latex bath serves to form covalent bonds bothwithin the individual rubber particles and between coalesced rubberparticles. A problem with vulcanization both at this stage and prior tothe dip is that the outer surfaces of the particles have greaterexposure to the vulcanizing agents than the particle interiors,resulting in a case-hardening effect and a lack of uniformity in therubber.

In dip-molding processes for rubber lattices, sulfur is often used as avulcanizing agent, although various accelerators, activators, sulfurdonors, and boosters are frequently included as well to optimize theprocess. The application of sulfur forms sulphidic cross-links betweenelastomer chains. In sulfur-based curing systems, the avoidance ofreversion and toxicity are often considerations.

An alternative means of prevulcanization of latex by free radicalcrosslinking involves the use of organic peroxides and hydroperoxides.Latex that is prevulcanized with these materials is referred to as“peroxide vulcanized natural rubber latex” (PVNRL). Peroxide curingforms carbon-carbon bonds. Utilization of this process on a commercialscale requires large and expensive heated pressure vessels, andprevulcanization is a necessary part of the process.

“Continuous vulcanization in liquid baths” (LCM Vulcanization) is usedon extruded rubber profiles. In LCM Vulcanization, a solid constantprofile shape is extruded, then submerged in a hot liquid bath such asmolten salt, hot oil, or melted lead, or in a hot fluid medium such asfluidized sand particles. Essentially all molecular oxygen is excludedfrom the curing environment. The use of the hot liquid bath or fluidmedium is to provide very rapid heat transfer rates to thin-wallextruded rubber profiles. U.S. Pat. Nos. 6,569,375; 6,775,848; and U.S.Pat. No. 6,920,643 to McGlothlin et al., the entire disclosures of whichare incorporated herein, address methods of vulcanization of dip-moldedrubber articles with molten media baths.

Certain nitrosamines are carcinogenic and highly undesirable in dipmolded products. U.S. Pat. No. 7,374,711 to McGlothlin et al., theentire disclosure of which is incorporated herein, addresses anaccelerator-free vulcanization process to produce both carbon-sulfur andcarbon-carbon crosslinking bonds. The vulcanization process beingperformed in the absence of any compounding components that containsecondary amine groups or any components that have a tendency to producenitrosamines.

Molten media baths work well, but commercialization for large, highvolume dip-molded goods such as condoms and gloves is difficult. Curingtimes as long as nine minutes or more require very long and large moltenbaths, which increase costs and/or create logistical issues formanufacturing facilities.

SUMMARY OF THE INVENTION

It has now been discovered that dip-molded articles of rubber need onlybe partially cured in a molten media bath. For example, latex is heatedby immersion of a dip former into a heated liquid media bath to bring itto a desired curing temperature; e.g., 350° F. The molten media bath canbe hotter than these temperatures to speed up the rate of temperatureincrease to get the latex to this desired curing temperature. Mostpreferably, the latex is not left in the bath any longer than to get tothe desired curing temperature and not to fully cure the latex. Once thelatex is at the desired curing temperature, it is immediately removedfrom the molten media bath and placed in a substantially oxygen freeheating oven to complete curing. Alternatively, upon removal from themolten media bath, the latex is quenched in a quenching bath. Anyunreacted peroxide in the latex is then extracted after separation fromthe dip former. Preferably, the molten media bath is free of nitrites.

In an embodiment of the invention, a method for the preparation of anarticle of rubber, the method comprises: dipping a forming member in alatex; withdrawing the forming member from the latex in a manner toleave a film of the latex over an outer surface of the forming member;immersing the forming member with the latex film thereon in a chemicallyinert liquid bath, for a duration of time, to partially cure and heatthe latex film to a predetermined temperature, wherein the duration oftime is less than a time it would take to fully cure the latex film;withdrawing the forming member from the chemically inert liquid bath;placing the forming member with the partially-cured latex film into asubstantially oxygen free oven maintained at or above the predeterminedtemperature to fully cure the latex film; and separating the cured latexfilm from the forming member. The chemically inert liquid bath is anitrite free salt bath. The latex can comprise a rubber and an organicperoxide, wherein the organic peroxide is dicumyl peroxide oralpha-bis(t-butylperoxy)diisopropylbenzene. The duration of time is lessthan or equal to six half-lives of the organic peroxide, four half-livesof the organic peroxide, or one half-life of the organic peroxide. Thepredetermined temperature is 350° F. plus or minus 25° F.

In another embodiment of the invention, a dip-molded article of a rubberis formed by a method comprising: dipping a forming member in a latex;withdrawing the forming member from the latex in a manner to leave afilm of the latex over an outer surface of the forming member; immersingthe forming member with the latex film thereon in a chemically inertliquid bath, for a duration of time, to partially cure and heat thelatex film to a predetermined temperature, wherein the duration of timeis less than the time it would take to fully cure the latex film;withdrawing the forming member from the chemically inert liquid bath;placing the forming member with the partially-cured latex film into asubstantially oxygen free oven maintained at or above the predeterminedtemperature to fully cure the latex film; and separating the cured latexfilm from the forming member. The chemically inert liquid bath is anitrite free salt bath. The latex can comprise a rubber and an organicperoxide, wherein the organic peroxide is dicumyl peroxide oralpha-bis(t-butylperoxy)diisopropylbenzene. The duration of time is lessthan or equal to six half-lives of the organic peroxide, four half-livesof the organic peroxide, or one half-life of the organic peroxide. Thedip-molded article is a surgical glove, examination glove, or condom.

In yet another embodiment of the invention, a dip-molded article of arubber is formed by a method comprising: dipping a forming member in alatex comprising a rubber and an organic peroxide; withdrawing theforming member from the latex in a manner to leave a film of the latexover an outer surface of the forming member; immersing the formingmember with the latex film thereon in a chemically inert liquid bath,for a duration of time, to partially cure and heat the latex film to apredetermined temperature, wherein the duration of time is less than thetime it would take to fully cure the latex film; withdrawing the formingmember from the chemically inert liquid bath; quenching the formingmember with the partially-cured latex film; separating the cured latexfilm from the forming member; and extracting residual and unreactedorganic peroxide from the separated cured latex film. The chemicallyinert liquid bath is a nitrite free salt bath. The organic peroxide isdicumyl peroxide or alpha-bis(t-butylperoxy)diisopropylbenzene. Theduration of time is less than or equal to six half-lives of the organicperoxide, four half-lives of the organic peroxide, or one half-life ofthe organic peroxide. The dip-molded article is a surgical glove,examination glove, or condom. The predetermined temperature is 350° F.plus or minus 25° F.

Among the many advantages of this invention is that completion of curingis facilitated in a simpler, less expensive hot air (gas) oven (or via aquenching bath and peroxide extraction process) as opposed to a lengthymolten media bath. This makes it much easier to convert an existingglove or condom production line into one that can process peroxide curematerials.

The foregoing, and other features and advantages of the invention, willbe apparent from the following, more particular description of thepreferred embodiments of the invention and the claims.

DETAILED DESCRIPTION OF EMBODIMENTS

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below.

The liquid bath, e.g., molten media bath, in which the dip former andfilm are immersed subsequent to the dip stage of the process, is aheated liquid that provides rapid heat transfer to the film. Furtherproperties of liquid media that are most desirable and thereforepreferred for this purpose are the lack of a tendency to migrate ordiffuse into the film on the dip former (unless the medium itself is adesirable constituent of the film), the quality of being stable withrespect to the surrounding environment (both the atmospheric environmentand the rubber-forming material as well as the various species that maybe compounded with the material), and the quality of remaining liquid atthe vulcanization temperature. Examples of liquid media that can be usedfor this purpose are molten inorganic salts, oils, glycols, liquifiedmetals, water, and brine solutions. Preferred among these are molteninorganic salts, silicone oils, and glycols, and the most preferred aremolten inorganic salts. Examples of suitable molten inorganic salts arenitrates, carbonates, sulfates, phosphates, and halides of potassium,sodium and lithium, as well as combinations of salts of this group. In apreferred embodiment of the invention, a liquid salt bath comprisingpotassium nitrate and lithium nitrate is used.

As noted above, some nitrosamines are carcinogenic and highlyundesirable in dip molded products. Accordingly, it is preferable to usea molten media bath that contains no nitrites. It was previously thoughtthat the use of organic peroxides instead of traditional rubberaccelerators would prevent the formation of nitrosamines. Moreover, thelatex dipping industry generally assumed that the elimination ofsecondary amine-containing accelerators and other compounding chemicalsprevented the formation of nitrosamines in dipped rubber articles.However, it has been discovered that these assumptions were incorrect;detectable levels of nitrosamines were found to be present in films whennitrites were present in the liquid bath. Accordingly, in a preferredembodiment to the invention, nitrites are not included in the liquidbath.

This invention is applicable to a wide range of rubber and rubbersubstitute compositions, including both lattices and organic solutions,thermoplastic elastomers, and thermoplastic polyurethanes.

Of the lattices, the one most commonly used is natural rubber. Naturalrubber can be obtained from several sources, including Heveabrasiliensis, Parthenum argentatum (commonly known as “guayule”),Taraxacum officinale, and Ficus elastica rubber trees. Natural rubberlatex is available in several grades, including high ammonia latex, lowammonia latex, and others. All such varieties are suitable for use inthe present invention. This invention also extends to natural rubberlattices that have been processed to reduce the amount of proteinspresent in the lattices. Some of these processes include centrifuging toseparate and remove water, and others include double centrifuging, inwhich an initial centrifuging is followed by the addition of water and asecond centrifuging. Still other processes involve the use of enzymes todigest the proteins.

Synthetic rubber lattices in general are likewise usable in the practiceof this invention. Examples are polyisoprene, ethylenepropylene-dieneterpolymer, styrene isoprene rubber, styrene butadiene rubber, styreneisoprene butadiene rubber, polybutadiene rubber, polychloroprene,nitrite rubber, styrene block copolymers, silicone rubber, and butylrubber. This invention also extends to polymer dispersions that are usedin a manner similar to rubber lattices. One example is an aqueousdispersion of a polyurethane thermoplastic elastomer. For thesedispersions, embodiments of the present invention that use curingsystems other than those that are sulfur-based can be used. Polyurethaneproducts such as medical examination gloves that are formed by theprocess of this invention exhibit increased resistance to solvents.

In addition to lattices and polymer dispersions, the present inventionalso applies to organic solutions. The organic solvents used in formingthese solutions are any solvents that are inert to the rubber, rubbersubstitute or polymer, and that are readily removable from thedip-molded film by evaporation. The solvent is preferably an aliphatichydrocarbon, saturated or unsaturated, linear, branched or cyclic, orethers, esters, alcohols or amines. Typical solvents are aliphatichydrocarbons containing five to eight carbon atoms, such as pentane,pentene, hexane, heptane, cyclohexane, and cyclopentane, andheterocyclic compounds such as tetrahydrofuran.

A wide variety of vulcanizing agents can be used in the practice of thisinvention. Useful vulcanizing agents include organic peroxides,sulfur-containing compounds, selenium-containing compounds, andtellurium-containing compounds. Organic peroxides, for example, may beused singly or in combination, and the most common types are diacylperoxides, peroxyketals, and dialkyl peroxides. Preferred organicperoxides are the dialkyl peroxides, particularly dicumyl peroxide.Other useful dialkyl peroxides are 2,5-dimethyl-di-(t-butylperoxy)hexane; di-t-butylperoxide; t-butylcumyl-peroxide;bis(t-butylperoxyisopropyl)benzene; butyl4,4-bis(t-butylperoxy)valerate;2,5-bis(t-butylperoxy)-2,5-dimethylhexane; 2,5-bis(t-butylperoxy)-2,5-dimethyl-3-hexane; t-butyl3-isopropenylcumylperoxide; bis(3-isopropenylcumyl) peroxide;1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane;t-butylperoxybenzoate, and bis(2,4-dichlorobenzoyl) peroxide.

In addition to dicumyl peroxide,alpha-bis(t-butylperoxy)diisopropylbenzene (“Alpha”) is a preferredperoxide. The advantage of Alpha as compared to dicumyl peroxide is thatit produces less odiferous off gas products in the cured latex films.Dicumyl peroxide leaves behind acetophenone, which has a strong odor.The use of hot air after a salt bath allows for efficient “airing out”of the peroxide breakdown products.

The present invention is very useful for peroxide curing in that itmakes it commercially feasible to cure latex films. For dip-molded partsthat do not contain peroxides, the present invention is also useful.Most accelerator/sulfur hot air cured films have less than desirablelatex particle integration and have poor uniform crosslinking. Whenfilms without peroxide are cured in a molten salt bath, the particleintegration and uniformity of cross linking throughout the cured film isfar superior to that of film just cured in a hot air oven. The moltensalt bath essentially melts the particles together while curing. Byusing just a hot air oven, the particles are just sintered or fusedtogether on their surfaces. Others have gone to great pains to optimizethe particle integration and uniform crosslinking in latex films. U.S.Pat. Nos. 8,087,412; 8,464,719; 9,074,027; and 9,074,029, all of theentire disclosures of which are incorporated herein by reference, areexamples that require meticulous attention to process details and stillare not as effective in integrating latex particles as is the moltenmedia batch process.

Coagents and other additives are often used in conjunction with theorganic peroxides to achieve products having particular properties.Certain coagents also add to the crosslinking efficiency of theperoxides by causing a single peroxide radical to produce more than onecarbon-carbon crosslink. Coagents can also be integrated into thepolymer network by covalent bonds to enhance certain properties of thepolymer, such as elongation and tear strength. Some of these coagentsare based on acrylate and methacrylate chemistry. All, however, aresuitable for inclusion in the methods and products of the presentinvention. Examples of suitable coagents are multifunctional salts ofacrylic and methacrylic acids. Of this group of coagents, SARET 634(whose primary ingredient is zinc dimethacrylate) and SARET 521 (whoseprimary ingredients are difunctional acrylate esters) are the mostpreferred. Trimethylolpropane trimethacrylate is another example.

Sulfur-based vulcanization systems include both small sulfur-containingmolecules and sulfur-containing polymers. Examples of sulfur-basedvulcanization chemicals are: mercaptothiazoles, for example,2-mercaptobenzothiazole and its salts, notably its zinc salt; thiuramsulfides and disulfides, for example, tetraethylthiuram monosulfide,tetrabutylthiuram monosulfide, tet-ramethylthiuram disulfide, andtetraethylthiuram disulfide; guanidines; thiourea and substitutedthioureas; thiocarbanilides and substituted thiocarbanilides, forexample, o-dimethyl-thiocarbanilide and its isomers and alkyl homologs;zinc alkyl dithiocarbamates, for example, zinc dimethyl dithiocarbamate,and accelerators containing these materials; sodium or potassiumdimethyl dithiocarbamate selenium dialkyl dithiocarbamates, for example,selenium diethyldithiocarbamate; 2-benzothiazyl-N,N-diethylthiocarbamylsulfide xanthates such as dibutyl xanthogen disulfide and xanthogenpolysulfide; alkyl phenol sulfides; dipentamethylene tetrasulfide;sulfur-containing polymers such as Thiokol VA-3; 4,4-dithiomorpholine;and miscellaneous disulfides such as bensothiazyl disulfide and bis(dimethylthiocarbamoyl) disulfide.

When the dip-molded articles of this invention are intended for use incontact with human skin, the preferred compounding ingredients are thosethat produce films that are biocompatible. Examples of compoundingingredients that serve this purpose for sulfur-vulcanized systems arexanthogen compounds such as diisopropyl xanthogen polysulfide,dibenzyldithiocarbamate, and higher alkyl zinc dithiocarbamates. Forperoxide vulcanized systems, a preferred compounding ingredient isdicumyl peroxide or Alpha. Again, Alpha does not produce many malodorousoff gases in comparison to dicumyl peroxide.

Reinforcing agents and other additives are also included in someembodiments of the invention. Examples of reinforcing agents are fumedsilica, carbon black, graphene, graphene oxide, and chopped fibers. Theuse of cut fibers improves the tear strength of medical gloves and theuse of fumed silica improve the tear strength of dipped films.Antioxidants and antiozonants may also be included to protect againstenvironmental aging. Pigments and dyes may also be included, as may anyof the other additives known to those skilled in the art of theformulation and manufacture of rubber devices.

An illustrative procedure for latex dip molding and curing in accordancewith the present invention is as follows:

1. Either a natural rubber or a synthetic rubber latex is compoundedwith vulcanizing agent(s) and possibly an antioxidant, a stabilizer orboth. If organic peroxide vulcanization is used, it will often besufficient to simply add to the latex a dispersion that contains anorganic peroxide.

2. Prevulcanization of the latex at this stage is optional and notrequired for all embodiments of this invention. When used,prevulcanization can improve the wet gel strength.

3. A dip former is optionally coated with a chemical coagulant bydipping the former into a bath of a coagulant-containing liquid, thenwithdrawing the former and drying it.

4. The dip former, with or without the coagulant coating, is dipped in abath filled with the compounded latex.

5. The dip former is slowly withdrawn from the bath. If the former had acoagulant coating, it now has a wet latex gel on its surface. If nocoagulant coating was applied, the former will have a liquid latex filmon its surface.

6. Excess water in the latex film on the dip former surface is removed,generally by evaporation in a hot air convection oven with either sweepgas or a partial vacuum. The process can be supplemented with infrared,microwave, or radiofrequency radiation, or any other type of energy toexpedite the evaporation. Vacuum drying is advantageous since it avoidsthe need for exposure of the dried latex to air at an elevatedtemperature prior to vulcanization.

7. The latex is heated by immersion of the dip former into the heatedliquid media bath to bring it to a desired curing temperature. Apreferred curing temperature range for the full scope of this inventionis about 235° F. to 600° F. For polychloroprene and styrenebutadienerubber, a preferred temperature range is about 300° F. to about 600° F.,while for natural rubber a preferred temperature range is about 235° F.to 500° F. The molten media bath can be greater than these temperaturesto speed up the rate of temperature increase to get the latex to thedesired curing temperature; e.g., 350° F. Experiments have shown thatthis can be done in under about one minute time. Most preferably, thelatex is not left in the bath any longer than to get to the desiredcuring temperature and not to fully cure the latex. The presentinvention exposes the latex to the media bath for a duration much lessthan prior art systems. For example, prior art methods leave the latexin the media bath for nine minutes at 350° F. to achieve a completecure. Here, the molten media bath only partially cures the latex. Wherethe latex includes an organic peroxide, the duration of time in themolten salt bath is measured in half-lives of the organic peroxide. Forexample, at a temperature of 350° F., dicumyl peroxide has a half-lifetime of approximately one minute. In a preferred embodiment of theinvention, the duration of time is less than or equal to one half-life.In another embodiment of the invention, the duration of time is lessthan or equal to four half-lives. In yet another embodiment of theinvention, the duration of time is less than or equal to six half-lives.In another embodiment of the invention, the duration of time is lessthan nine half-lives.

8. Upon the latex being brought up to the desired curing temperature viathe molten media bath, the dip former with the partially-cured latexfilm is immediately removed from the bath and its temperature ismaintained at the desired curing temperature in a low/no oxygen heatingoven for a period of time sufficient to react ninety (90) percent ormore of the organic peroxide. The oven can be any type of oven such as ahot gas oven, a heated chamber, a plenum, or any type of enclosurecontaining a low oxygen or oxygen free heated environment. In otherwords, the period of time is equal to about nine (9) half-lives of theperoxide (minus the time spent in the molten bath), which eliminatesvirtually all of the peroxide as shown in the following table.

Half-Lives % Cured 1 50.0 2 75.0 3 87.5 4 93.8 5 96.9 6 98.4 7 99.2 899.6 9 99.8 10 99.9

The net effect of this step is to very substantially reduce the time inthe salt bath by a significant amount; e.g., ninety (90) to ninety-five(95) percent or more. In embodiments of the invention, the low/no oxygenenvironment is facilitated via a “zero air” gaseous environment or inertgas environment such as, but not limited to helium, nitrogen, and/orargon, or reduced oxygen air. Nitrogen is preferred due to its wideavailability. For low oxygen air, the flue gas from natural gas orpropane combustion can be used both to maintain temperature and toeliminate oxygen. A low oxygen environment is one without enough oxygento significantly react with any remaining peroxide.

9. After curing of the latex, the rubber articles are removed from theoven and cooled; for example, in air or a stream of water. Water may beused to rinse off any excess solidified heat transfer medium such assolidified salt.

10. The vulcanized latex article is manually or mechanically strippedfrom the dip former.

In an alternative embodiment of the invention, curing time is reduced byactively quenching the dip former with latex film before curing iscomplete. Quenching can be an alternative to the low/no oxygen heatingoven above entirely (or to reduce the time in the heating oven).Quenching eliminates atmospheric oxygen from reacting with residualperoxide, which creates tackiness via oxidation of the rubber. Forexample, a 2.4 phr of dicumyl peroxide formulation can be cured for oneminute, which is the half-life of dicumyl peroxide at 350° F. Rapidlyquenching the reaction of the formulation (when heated) after one minuteprovides a fully cured film, but with a residual level of 1.2 phr ofunreacted dicumyl peroxide in the rubber, which is not desirable. Afterthe dipped goods are stripped, residual unreacted dicumyl peroxide isextracted via a soxlet extraction method, a critical CO₂ extraction,ethyl acetate soaking method, or a rubber extraction method, theidentification and implementation of all of which are apparent to one ofordinary skill in the art.

In another embodiment of the invention, a layer of molten salt isallowed to stay on the outside of the rubber film to form a barrierbetween the oxygen containing environment and the still-curing rubberfilm. If desired, it would be further possible to heat the molten saltbath and the dip former to a temperature well above the normal curingtemperature of the rubber and to then leave on a molten salt film orplacement of the former/mandrel into a low/no oxygen heating ovenwithout actually bringing the new environment up to the normal 350° F.temperature. As the “overheated” former cools in the lower temperatureatmosphere, the temperature is still high enough to allow for completecuring of up to about nine (9) half-lives.

An illustrative procedure for solvent dip molding and curing inaccordance with the present invention is as follows:

1. Solid granules of synthetic or natural rubber elastomer are dissolvedin a suitable solvent to form a cement. Suitable compounding agents aredispersed or dissolved in the cement. Compounding agents similar tothose used in the latex processes, including organic peroxides, can beused.

2. No prevulcanization is necessary, as all compounding agents areuniformly dispersed in the cement. The cement is placed in a dip tank,and a dip former is dipped in the cement.

3. The dip former is slowly withdrawn from the dip tank to leave a filmof the cement over the surface of the dip former.

4. Solvent is evaporated from the dip former to leave a uniform polymerfilm on the surface. Removal of the solvent can be achieved by ambientor hot air drying.

5. The latex is heated by immersion of the dip former into the heatedliquid media bath to bring it to a desired curing temperature. Apreferred curing temperature range for the full scope of this inventionis about 235° F. to 600° F. For polychloroprene and styrenebutadienerubber, a preferred temperature range is about 300° F. to about 600° F.,while for natural rubber a preferred temperature range is from about235° F. to 500° F. The molten media bath can be greater than thesetemperatures to speed up the rate of temperature increase to get thelatex to the desired curing temperature; e.g., 350° F. Experiments haveshown that this can be done in under about one minute time. Critically,the latex is not left in the bath any longer and not to fully cure thelatex.

6. Upon the latex being brought up to the desired curing temperature viathe molten media bath, the dip former with the partially-cured latexfilm is immediately removed from the bath and its temperature ismaintained at the desired curing temperature in a low/no oxygen heatingoven for a period of time sufficient to react ninety (90) percent ormore of the organic peroxide as noted above.

7. After curing of the latex, the dip former is withdrawn from the ovenand cooled in air or a stream of water.

8. The dip former is then soaked in water to help break the adhesionbetween the film and the dip former.

9. The vulcanized latex article is manually or mechanically strippedfrom the dip former.

While the present invention virtually eliminates the need forprevulcanization and maturation of the compounded latex or solution,prevulcanization is useful with lattices that would otherwise have anexceptionally low wet or dry gel strength. Prevulcanization can be doneby any conventional method. Such methods include, but are not limitedto, sulfur prevulcanization, peroxide prevulcanization, andprevulcanization by high energy irradiation, all of which may beperformed as they are in the prior art. Good wet gel strength is usefulin preventing cracks from forming in the film as the film is beingdried. In the case of natural rubber, both wet and dry gel strengths aregenerally adequate without prevulcanization. The gel strengths of somesynthetic latices are lower, however, and prevulcanization may improvethe processing, but is not essential. Prevulcanization by high energyirradiation can also serve to reduce the amount of vulcanizationchemicals needed and hence the levels of undesirable residual chemicalsin the final product.

It is often useful to determine the extent to which a dipped film orarticle has been vulcanized. A commonly used method is to cut out acircular disk of the cured film and measure the change in diameter uponimmersion of the disk in a solvent bath. Similar test methods areavailable for other types of vulcanized polymers, and are well known tothose skilled in the art.

The following examples are offered for purposes of illustration, and arenot intended to limit the scope of the invention.

Example 1 Process According to the Invention: Natural Rubber Latex

This example illustrates the process of the present invention. Acoagulant solution in ethanol was used, containing approximately twenty(20) percent calcium nitrate, and 0.5 percent Igepal C0-630, all byweight, the balance denatured ethanol. To 1 kg of natural rubber latexwas added 19.5 g of the dicumyl peroxide emulsion, and the resultingcomposition was mixed under medium shear for thirty minutes on alaboratory mixer. In addition, fumed silica was added at 2 phr in theform of a fifteen (15) percent (by weight) aqueous dispersion. Afterthirty minutes of mixing, the solution was rolled for thirty minutes ona laboratory roll mill, then degassed for ten minutes at 0.3 atmosphereabsolute pressure. This yielded approximately 1 liter of natural rubberlatex formulated with 1.3 phr dicumyl peroxide. The glass former wasdipped into the coagulant solution, then dried for five minutes at 40°C., then slowly dipped into the formulated latex where the former wasallowed to dwell for five seconds. The former was then slowly withdrawnand dried at 60° C. for sixty minutes. Once dried, the former and itsadherent film were immersed in a molten salt bath for one minute at 350°F. (177° C.) and then placed into a low/no oxygen heating oven tocomplete curing. The film was then rinsed, stripped, and readied fortensile testing. The film appeared translucent-to-clear and slightlyamber in color and more transparent than many sulfur-vulcanized rubberfilms. The film produced in this example is expected to meet thenecessary tensile strength requirements for both surgical gloves andcondoms.

Example 2 Process According to the Invention: Polychloroprene

This example illustrates the process of the present invention as appliedto polychloroprene, using procedures similar to those of the precedingexamples. The polychloroprene was a latex containing weight percentsolids, and is sold commercially as NEOPRENE 750.

A dicumyl peroxide emulsion was added to the latex to attain aformulated latex containing 0.1 phr dicumyl peroxide. Also added to thelatex was fumed silica (reinforcing agent), added as a fifteen (15)weight percent aqueous dispersion to achieve a level of 3 phr fumedsilica.

The glass former was first dipped into an aqueous coagulant solution,which contained 35% calcium nitrate, 0.5% IGEPAL C0-630 surfactant, bothby weight, the balance dionized water, then allowed to dry. The formerwas then dipped in the compounded latex and allowed to dwell in thelatex for five seconds, then slowly withdrawn and dried at 60° C. forsixty minutes. After drying, the former with latex film was immersed ina molten salt bath having the same composition as the baths used in thepreceding example, for one minute at 350° F. (177° C.) and then placedin a low/no oxygen heating oven to complete curing. The former and filmwere then rinsed, stripped, and readied for tensile testing. Theresultant latex film was transparent and amber in color, and sufficientto pass the ASTM standard D-3577-98 for synthetic rubber surgicalgloves.

Example 3 Process According to the Invention: Polyurethane

This example illustrates the process of the present invention as appliedto polyurethane, and specifically, in the modification of thermoplasticpolyurethane films after the films have been formed.

Two solvent dip molding solutions were prepared. The first consisted of(fifteen) (15) weight percent thermoplastic polyurethane and eighty-five(85) weight percent tetrahydrofuran. A control film (in the form of acondom) was prepared by dipping the form into an organic solution, asdescribed in U.S. Pat. No. 4,954,309, to McGlothlin et al., the entiredisclosure of which is incorporated herein by reference. After drying,the polyurethane condom thus formed was stripped from the former. Thesecond dip molding solution was formed by adding 0.5 phr dicumylperoxide to the first solution, and a second dip-molded condom wasprepared in a manner essentially identical to the first, except that thedipped and dried condom was then immersed for one minute in a moltensalt bath (identical to those used in the preceding examples) at 350° F.(177° C.) and transferred to a low/no oxygen heating oven to completecuring.

Portions of both the control condom and the test condom were subjectedto a solvent resistance test. According to this test, both films wereimmersed in tetrahydrofuran. The control film dissolved entirely whenimmersed in the tetrahydrofuran, while the second, which had beencrosslinked by the dicumyl peroxide treatment, did not dissolve butinstead swelled significantly. This test illustrates the improvement inproperties of dip-molded articles made of polyurethane (asrepresentative of thermoplastic elastomers in general) as a result ofthe process of the present invention.

Example 4 Process According to the Invention: Prevulcanized NaturalRubber Latex

This example illustrates the process of the present invention applied totwo prevulcanized natural rubber lattices, one by sulfur and the otherby radiation. The sulfur prevulcanized latex was sixty (60) percentsolids REVULTEX HLA-21. The radiation-prevulcanized latex was “RVNRL.”Both lattices are noted for their low levels of residual chemicals andhence their low toxicity profiles. Because of the low toxicity profiles,the tensile strengths of these materials are lower than those of manyother natural rubber lattices. Standard clear-glass condom formers, 32mm in diameter, as used in all preceding examples were used as dipformers. Four compounded latices were used, as follows:

1. REVULTEX HLA-21 (sulfur-prevulcanized latex) as supplied by RevertexAmericas.

2. REVULTEX HLA-21 (sulfur-prevulcanized latex) as supplied by RevertexAmericas, supplemented with dicumyl peroxide to 1.0 phr.

3. RVNRL as supplied by Guthrie Latex, Inc.

4. RVNRL as supplied by Guthrie Latex, Inc., supplemented with dicumylperoxide to 1.0 phr.

One condom was formed from each of these three lattices, using thecoagulant solution and the dipping and drying procedures of Example 3.All were then dried for sixty minutes at 60° C. The condoms formed fromlattices that did not contain dicumyl peroxide were further dried for 45minutes at 150° F. (66° C.) in a hot air oven, powdered, stripped, andset aside. The condoms formed from lattices that did contain dicumylperoxide were further processed by immersion in a molten salt bath ofthe same description as those used in the preceding examples, for oneminute at 350° F. (177° C.) and placed low/no oxygen heating oven tocomplete curing. All four condoms were rinsed, powdered, and stripped.The properties of the dip-molded condoms of both methods ofprevulcanization, sulfur-based and radiation, are enhanced bypostvulcanization in accordance with the present invention.

Example 5 Process According to the Invention: Addition of VulcanizingAgent by Imbibition for Secondary Postvulcanization

This example illustrates that aspect of the present invention in which avulcanized and fully formed dip-molded article is given a secondarypostvulcanization by first immersing the article in a solution of avulcanizing agent to absorb the agent from the solution and thenre-curing the article following the absorption. The rubber material usedin this example was synthetic polyisoprene rubber, supplied as a ten(10) percent solids solution in n-hexane. The polyisoprene was NATSYN2200, and was dissolved in the hexane by agitating with a medium-shearlaboratory mixer. The resulting solution was split into two batches, andthe first was supplemented by the addition of dicumyl peroxide to 1.5phr while the second was supplemented by the addition of dicumylperoxide to 2.0 phr. Stainless steel dipping mandrels with outsidediameters of 0.091 inch (0.23 cm) were dipped in the solutions,withdrawn, air dried and re-dipped in a sequence that was repeatedapproximately seven times to build up a single wall balloon thickness ofapproximately 0.010 inch (0.0254 cm). After thorough drying in a warmair oven to remove essentially all of the solvent, the portions of thedipping formers that were coated with the dried mixture of polyisopreneand dicumyl peroxide were immersed in a hot molten salt bath (the sameas that used in the preceding examples) for one minute at 350° F. (177°C.) and placed in a low/no oxygen heating oven to complete curing. Theresulting balloons were rinsed in water, powdered with corn starch, andremoved from the dipping formers. Each balloon was then cut intosegments approximately 1 cm in length to form right heart catheterballoons.

Six of the balloons formed from the 1.5 phr dicumyl peroxide dippingsolution were immersed for thirty minutes in an imbibing solutionconsisting of dicumyl peroxide dissolved in ethyl acetate, the solutionhaving a sufficient concentration and volume to raise the dicumylperoxide content of the balloons by 0.5 phr. The balloons were thenremoved from the solution and thoroughly air-dried in a warm air oven toremove essentially all ethyl acetate. The balloons were then immersed ina molten salt bath (as described in the preceding examples) for oneminute at 350° F. (177° C.) and placed in a low/no oxygen heating ovento complete curing. The balloons were then removed, rinsed in water,dried, and powdered with corn starch. An unexpected improvement inphysical properties was achieved by a two-stage postvulcanizationachieved by the imbibition of a vulcanizing agent by an already-formeddip-molded rubber article, followed by vulcanization in a hot liquidbath and oven, as compared to a single-stage postvulcanization at thesame level of vulcanizing agent.

Example 6 Comparison Using Natural Rubber Latex: Hot Liquid Medium CureAccording to the Invention Vs. Hot Air Cure of Prior Art

This example demonstrates the improvement offered by the presentinvention. Natural rubber latex supplemented with a sulfur-based curingsystem was used in this comparison.

Natural rubber latex (sixty percent (60%) solids) was supplemented witha curing system bearing the name OCTACURE 590 in an amount which,according to the supplier, results in a compounded latex containing 2phr zinc oxide, 1.65 phr sulfur, 0.5 phr zinc-2-mercaptobenzothiazole,and 0.75 phr of an unspecified antioxidant. The latex was degassed, andtwo condoms were prepared from the latex in the manner described inExample 1, involving the use of the coagulant described in that example.One of the condoms while still on the former was then vulcanized in hotair for 45 minutes at 100° C., and then for an additional sixty minutesat 110° C. The second condom, also while still on the former, was driedfor forty-five minutes at 100° C., then immersed in a molten salt bathof the same description as those used in the preceding examples for oneminute at 350° F. (177° C.) and then placed in a low/no oxygen heatingoven to complete curing. The present invention is applicable to naturallatex rubber without the need for prevulcanization.

Example 7 Application of the Invention to Latex Mixtures

This example demonstrates the application of the process of theinvention to a mixture of lattices.

A mixture was prepared by combining equal parts by weight of ShellIR-307 synthetic polyisoprene latex and NEOPRENE 750 polychloroprenelatex. A dicumyl peroxide dispersion was added to achieve a latexcontaining 0.7 phr dicumyl peroxide. One condom was produced from thislatex, using the method described in Example 1, then immersed in amolten salt bath of the same description as those used in the precedingexamples for one minute at 350° F. (177° C.), placed in a low/no oxygenheating oven to complete curing, then rinsed and powdered. The condomwas opaque and amber in color.

The foregoing is offered primarily for purposes of illustration. It willbe readily apparent to those skilled in the art that the materials andtheir proportions, as well as the operating conditions, procedural stepsand other parameters of the inventions described herein may be furthermodified or substituted in various ways without departing from thespirit and scope of the invention.

1. (canceled)
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 9. A partially-cured dip-moldedarticle of a rubber formed by a method comprising: dipping a formingmember in a latex; withdrawing the forming member from the latex in amanner to leave a film of the latex over an outer surface of the formingmember; immersing the forming member with the latex film thereon in achemically inert liquid bath, for a duration of time, to partially-cureand heat the latex film to a predetermined temperature, wherein theduration of time is less than the time it would take to fully cure thelatex film; and withdrawing the forming member from the chemically inertliquid bath.
 10. The partially-cured dip-molded article of claim 9,wherein the chemically inert liquid bath is a nitrite free salt bath.11. The partially-cured dip-molded article of claim 9, wherein the latexcomprises a rubber and an organic peroxide, wherein the organic peroxideis dicumyl peroxide or alpha-bis(t-butylperoxy)diisopropylbenzene. 12.The partially-cured dip-molded of claim 11, wherein the duration of timeis less than six half-lives of the organic peroxide.
 13. Thepartially-cured dip-molded article of claim 12, wherein the duration oftime is less than or equal to four half-lives of the organic peroxide.14. The partially-cured dip-molded article of claim 13, wherein theduration of time is less than or equal to one half-life of the organicperoxide.
 15. The partially-cured dip-molded article of claim 9, whereinthe partially-cured dip-molded article is a partially-cured surgicalglove, examination glove, or condom.
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